Is your kitty on any supplements or medication? Have you tried appetite stimulants? If you could get a copy of her latest lab results, that would also be helpful.
While putting in a feeding tube can be a life-saver in many situations, it should be more of a last resort thing, when other less invasive options have failed. And I also agree with mikischo that you don't want to go the low-protein route. That won't help at all in the weight gain department.
To get her eating more, you might have to try offering her a few different brands and flavours of wet food. Some other good ones to try besides Wellness are Innova Evo 95% venison or beef, Nature's Variety Instinct, and Precise has some good low phosphorus flavours as well. Does she like raw or lightly cooked chicken breast? That would be an excellent source of quality protein.
You can also top-dress her canned food with stuff like powdered freeze-dried meat treats (Halo Liv-a-Littles or Real Food Toppers, for example). Even pulverizing some of her favourite kibble and sprinkling it on top might help. Parmesan cheese or cat nip can also work.
Without knowing what you've already tried and what her blood work results are, I'd say that it's too early to go with a feeding tube, especially if it's just to force a completely inappropriate low-protein diet into her. How long is the vet thinking of keeping it in her? What happens when it comes out? It just doesn't make sense to me.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler